1. Field of the Invention
This invention relates generally to a holding device for a dual axis translation mechanism and in particular to an x,y translation mechanism useful in an x-ray spot film device which can accept film cassettes of varying width and length.
2. Description of the Prior Art
Conventional spot film devices usually comprise a support or frame extending crosswise over the top of an x-ray table. A main carriage is mounted on the support for being advanced from a rearward parked position to a frontward radiographic position wherein a film cassette carried by an inner carriage is disposed in alignment with an x-ray beam that is projected through a patient from a collimated x-ray source in the table. The film cassette is mounted or held in a tray supported on the inner carriage which is translatable with respect to the main carriage so that the center of the collimated x-ray beam may be made coincident with a selected area on the film cassette where one or a sequence of spot film exposures are to be made. The area is further defined by a superimposable x-ray mask. Due to differences in the size of the image to be made and the number of areas to be used on the film, different size films and film cassettes are available for use in the spot film devices.
As is well known, spot film devices are also used in conjunction with a fluoroscopic device which permits an examining radiologist to visualize anatomy of interest before making one or more radiographs in a choice of sizes by translating the film cassette forward and shifting it, and the mask, to obtain the desired sequence of radiographs. The fluoroscopic device on the spot film device is aligned with the x-ray source in the table, and the film cassette is, of course, retracted from the x-ray beam during fluoroscopy.
When a fluoroscopic view of interest is observed, the film cassette must be positioned into the x-ray beam path rapidly and one or more exposures must be taken while the fluoroscopically observed condition persists.
The x,y translating mechanism of spot film devices generally employs multiple electromechanical means for receiving varying sizes of film cassettes and rapidly advancing and retracting the film cassette between load and parked positions and between parked and the various positions in which the sequence of radiographs are taken. In addition, means are provided for predetermining the sequence and for cushioning the shock forces that are incidental to rapid transfer of the film cassette carriage from the parked position to its other positions when alternating between fluoroscopic and radiographic or loading modes. Furthermore, the spot film device must have some way of sensing the size of the inserted film cassette in order to properly control its above-noted movements.
These functions have been achieved in known translation mechanisms by complicated arrangements of mechanical linkages, multiple motors, tracks, cams, relays, belts and so forth, which accomplished their purposes under manual or mechanical influence or under a combination of such influences. Cassette translation mechanisms including motor drive means typically include two reversible motors, one motor for each of the orthogonal directions in which the film cassette must be driven. Additionally, one or two smaller motors are typically used in order to control grasping of differently sized film cassettes. The requirement of two or more motors undesirably increases the weight and power consumption of the drive system. In other types of motor drive systems having one and/or two motors, as a motor moves the film cassette rearwardly to a parked or home position, it slowly loads a return spring at the same time. The film carriage is latched in the parked position and when the latch is released, the carriage is advanced rapidly under the influence of the spring and halted abruptly in the radiographic position. Rapid movement and abrupt halt of the film carriage results in considerable noise, shock and vibration that necessitates use of shock absorbing devices such as dash pots to reduce these ill effects. One problem with this type of system is that the main carriage must be returned to a rearward position after each exposure, to reload the spring, after which the carriage must be projected forwardly again to make the next exposure.
Other types of prior spot film devices have a set of tracks for the main carriage. After each exposure, the carriage is returned rearwardly and shifted to different tracks, similar to railroad car switching. When the carriage is driven forwardly, it arrives in the proper position for the next exposure to be made. This is a relatively slow method and requires a large and complicated mechanism which has many moving parts.
Prior art cassette holding and size sensing mechanisms typically comprise motors or spring driven sliding and/or pivoting clamp arms for holding and engaging the film cassette and include potentiometers and/or encoders connected to the clamp arms for providing size sensing information signals.
The above complex arrangements have resulted in lower than desirable reliability and serviceability, increased power consumption, a massiveness that has had to be off-set with increased counterweight and/or power and a noisy operation. Some of the complexity and size resulted from spot devices being adapted to accommodate rectangular rectangular cassettes in both their long and short dimensions in which case means had to be provided for altering the mode of operation of the transfer and sequence mechanism depending on how the cassette was oriented in its holder.
My prior U.S. Pat. No. 5,010,564 entitled DUAL AXIS TRANSLATION MECHANISM relates to a translation mechanism particularly useful in a spot film device that is simple in construction, lightweight, operates quietly, efficiency, safely, and automatically, is simple to operate and maintain and is comparatively inexpensive to manufacture. More specifically, my above-noted patent discloses a mechanism for selective translation of an object, such as a film cassette, along first and second axes, comprising a frame, a carriage mounted on the frame for movement with respect to the frame along a first axis, an object holder for holding said object and an intermediate mounting means coupled to the object holder and the carriage for mounting the object holder on the carriage for movement with respect to the carriage along a second axis which is different from the first axis. A motor is provided for selectively driving a first shaft. A drive/brake assembly provides a second shaft which, in a drive mode of the assembly is caused to be driven synchronously with the first shaft and, which in a brake mode of the assembly, is held non-driven while the first shaft is being driven. First and second endless belts are mounted for rotating movement within the frame and extend along the first and second shafts, respective, for selectively receiving rotational movement therefrom. The first belt is also coupled to the intermediate mounting means so as to drive a portion thereof for causing selective movement of the object holder along the second axis and the second belt is coupled with the carriage for causing selective movement of said carriage along the first axis. In operation, when both belts are synchronously rotated, the second belt causes the carriage to move along the first axis, while the intermediate mounting means does not cause movement of the object holder because of the synchronous movement of the intermediate mounting means with respect to the first belt, due to the coupling of the intermediate mounting means on the carriage. However, when the second belt is not driven during the brake mode of the drive/brake assembly, relative movement of said first belt with respect to said intermediate mounting means causes movement of the object holder along the second axis.
Controlled movement of the first and second shafts by the motor and drive/brake assembly results in the ability to rapidly position the object holder at any desired position along the first and second axes in a simple, efficient and reliable manner.
In a preferred embodiment, the drive/brake assembly comprises a clutch/brake assembly having an input shaft which is driven by the same motor which drives the first shaft.
The present invention is directed to incorporating a holder for variable size objects as well as an object size sensing mechanism into my above-described dual axis translation mechanism.